A. Field of Invention
The present invention is directed toward the field of signal synthesizers. More particularly, the present invention relates to a frequency reference for use in a signal synthesizer.
B. Description of the Related Art
A signal synthesizer is used to generate a signal or series of signals that have a specified set of characteristics. In one example, a frequency is specified for an output signal that is produced by the synthesizer. In another example, a range of frequencies is specified for a series of signals that are to be swept by the synthesizer. In order to provide a desired output frequency, the signal synthesizer employs a reference signal having a reference frequency. The reference frequency serves as a basis for deriving the desired output frequency.
Signals produced by the signal synthesizer are employed in a variety of applications, such as testing for faults in cables and other communication media. In many testing applications, it is desirable for the signal synthesizer to provide very accurate output frequencies at a high frequency resolution. When sweeping through a series of output signals, it is also important that the synthesizer has a small settling time, so that the output signals can be provided rapidly for use by the synthesizer""s operator.
It is very challenging to provide output signals with high resolution, accurate frequencies, and small setting times. In fact, the reference frequency itself introduces frequency errors, by randomly deviating from an ideal reference frequency that the synthesizer expects to receive. Frequency modification circuitry within the signal synthesizer, such as a frequency synthesizer, can also cause additional frequency errors.
In order to improve the resolution and accuracy of a signal synthesizer, a frequency synthesizer within the signal synthesizer can employ multiple frequency synthesis loops with large frequency divisor values. However, the use of multiple loops can, significantly increase the number of components used in the signal synthesizer. Such extra components can result in the cost and power requirements of the signal synthesizer being increased, which is especially undesirable in portable signal synthesizers. Additionally, the use of large frequency divisor values can cause settling times to increase significantly.
Accordingly, it is desirable to provide for a signal synthesizer that is able to accurately generate signals without compromising output frequency resolution or settling time or significantly increasing the number of components in the signal synthesizer.
In accordance with the present invention, a programmable frequency reference is provided in a signal synthesizer to enhance the synthesizer""s output signal frequency accuracy and resolution, without compromising settling time. A signal synthesizer having such a programmable frequency reference also includes frequency modification circuitry. The frequency modification circuitry is coupled to the frequency reference to receive a reference frequency and modify it to produce a signal synthesizer output frequency. The programmable frequency reference provides the reference frequency, so that the reference frequency offsets frequency error that is, introduced by the frequency modification circuitry. As a result, the accuracy and resolution of the signal synthesizer""s output frequency is improved, without adding circuitry to the frequency modification circuitry, or compromising the signal synthesizer""s settling time.
The frequency reference includes a voltage controlled oscillator, programmable voltage generator, and voltage control engine. The voltage controlled oscillator provides a signal having the reference frequency in response to a frequency setting voltage. The programmable voltage generator has an output coupled to the voltage controlled oscillator for providing the frequency setting voltage in response to a voltage setting value. The voltage control engine has an output coupled to the programmable voltage generator for providing the voltage setting value.
The voltage control engine sets the voltage setting value in response to the frequency error that is introduced by the frequency modification circuitry. The voltage setting value is set to provide for the generation of a frequency setting voltage that causes the voltage controlled oscillator""s reference frequency to have a certain deviation from the nominal frequency of the voltage controlled oscillator. The certain deviation is one that provides for offsetting the frequency error caused by the frequency modification circuitry. As a result, an accurate signal synthesizer output frequency is generated.
In one embodiment of the present invention, the frequency modification circuitry includes a frequency synthesizer and a frequency divider coupled to receive the output of the frequency synthesizer. The frequency synthesizer is coupled to receive the reference frequency and convert it into a synthesizer frequency that is divided by the frequency divider to produce the signal synthesizer output frequency. In such an embodiment, the frequency synthesizer provides the frequency error that is corrected by the programmable frequency reference.